'G20 heads of state, engage yourself finally against hunger in the world', declares an Action Against Hunger banner in Paris. Photograph: Jacques Demarthon/Getty

As western economies continue to flounder, the G20 meeting in Cannes next month has taken on epic importance in the minds of economists. Something has to happen there, western politicians insist, that will save the west, and particularly Europe, from its current deadlock.

There was a time, only a few short years ago, when the eight most powerful leaders in the world would meet to discuss how to demonstrate their largesse to the billions of people still living in extreme poverty. In Cannes, western leaders will be in the uncomfortable position of asking countries they have previously treated as worthy of their charity for a helping hand in making the world economy more palatable for a struggling euro.

But should they expect support from countries that have their own acute needs to take care of? There is no moral reason for taking a hit for the west, which has enjoyed unparalleled wealth for centuries, putting the wellbeing of much poorer countries far down its list of priorities. In the past few years, it has even lived well beyond its means. While no one can deny the tragedy of joblessness and home repossessions, the poverty in other countries around the world is far worse, and they should not be asked to put their economies at risk to help the west out of a hole.

Do poor countries have a self-interested economic case to help the rich? Barack Obama, the US president, said recently that Europe is "going through a financial crisis that is scaring the world", adding that "economies around the world are not growing as fast as they need to". What he meant, of course, was that the crisis is scaring the western world and that western economies are not growing as fast as they need to.

Had he looked at the actual world, he would have seen that things are going fairly well. Growth certainly dipped in most developing countries at the outset of the western financial crisis, but things have been back on track in the past couple of years, according to growth data. Overall, the past decade was one of the best in memory not only for the emerging markets but, crucially, for the low-income countries in Africa.

So do developing countries need Europe to be doing well if they too are to grow sustainably? No one knows (which is probably the most important thing to remember in economic research). But there doesn't appear to be a magic rule that what is good for the world's richest countries will also be good for its poorest countries, or emerging economies, or vice versa.

Politicians have to pretend such a rule exists as they smile and pose for photos. But growth in some parts of the world and not others is just as plausible as growth all over the place. Plenty of economists view the rapid growth of Europe and the US in the past two centuries as a cause of impoverishment in other countries, rather than an unrelated consequence of sound economic management and hard work.

While income per capita has grown rapidly in the rich world for the past three decades, from about $14,000 (constant 2000) to about $26,000, it declined in sub-Saharan Africa (from $571 to $507) and stagnated in all low-income countries (at around $240) until things started to get marginally better in the first decade of this century. It is simply not the case that western prosperity is necessarily associated with prosperity elsewhere.

With China and the other Bric countries (Brazil, Russia and India) providing markets for developing world exports in the short term, there is no immediate concern in much of the poor world. Nevertheless, continued growth in the emerging markets is by no means assured and it is certainly possible that a return to prosperity in the west could be good for the poorest. It depends entirely on whether the policies being pursued support growth elsewhere or undermine or neglect it.

There is every hope that in the new era of international relations, as even the poorest countries, not just the Brics, begin to assert themselves more, exploitation will be harder and mutually beneficial policies will indeed finally be implemented. But there is no guarantee of that. That is the task of the G20. The west will have to give up its habit of seeking the best for the west, and think about what is best for the rest.

As optimists, we can be hopeful. But as realists, we know tough times may make it even less likely that the kind of advantage sought by the west in its trade, financing and subsidising policies will be altered, and that progressive policies (for instance, on human rights and climate change) may be sacrificed on the altar of economic growth. The difference now is that poorer countries and their big brothers are less likely to give in and more likely to demand fundamental changes that will actually benefit them.